The present invention generally relates to data backup technology.
For example, Japanese Patent Application Laid-open No. H11-212875 discloses a method by which in a storage subsystem including a primary subsystem and a secondary subsystem, the primary subsystem performs full backup and differential backup of data received from a higher-level device in the secondary subsystem. Furthermore, a method for performing data restoration from the data backed up in the secondary subsystem is also disclosed.
For example, there exist methods by which a computer, a disk storage, and a tape library device are connected to a communication network termed SAN (Storage Area Network) and data stored in the disk storage are backed up to the tape library device by a server for processing of backup (referred to hereinbelow as “backup server”). In this case, the data that are the backup object are backed up via the backup server. The resultant problem is that the backup performance depends on the backup server, and when the processing performance of the backup server is low, the time required for the backup is increased.
A similar problem is also encountered, for example, when the backed-up data are restored in the processing flow that is inverted with respect to the backup, more specifically, when data are restored from the tape library device to the disk storage via the backup server.
Those problems can be resolved by the backup method disclosed in Japanese Patent Application Laid-open No. H11-212875. However, with this backup method, in the case of differential backup, data are recorded on a tape, and the information necessary to restore the data, for example, information for managing the difference between the data (referred to hereinbelow as “differential data information”) is stored in the control memory of the secondary subsystem. For this reason, the processing of data restore is performed by using both the data recorded on the tape and the differential data information stored in the control memory of the secondary subsystem. The resultant problem is that, for example, when the information of the control memory of the secondary subsystem is lost due to an accident or the like, the differentially back-up data cannot be restored.
Yet another problem is that in order to perform differential backup of multiple generations to store the differential data information in the control memory of the secondary subsystem, the control memory has to have a storage capacity enabling the memory to store the differential data information of each generation, and the number of generations for which the differential backup is performed is limited by the storage capacity of the control memory.
Such problem is not limited to differential backup and is also encountered in the case of a backup of a type such that restoration cannot be performed if the information that is necessary for the restoration (referred to hereinbelow as “information necessary for restoration”), such as differential data information, is absent (for convenience, such backup will be referred to hereinbelow as “partial backup”), for example, when an incremental backup is performed.